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Sewing up smooth linear motion without threads

Turn a smooth shaft. Butt a roller against it with axis skewed relative to the shaft. The roller wants to translate, and you have the essentials of a threadless mechanical linear actuator..

Tom Hughes of Power Transmission Design | Aug 01, 2000

All right, so it’s not quite so simple as that. Still, it works and several suppliers offer such units for a variety of applications. By adding means to adjust the size of the angle between shaft and roller axes, or to reverse the angle (and thus, linear-motion direction), you obviate the need to adjust motor speed or direction — the device becomes more versatile. Below are some of the best and worst situations for threadless mechanical linear actuators.

Take care. Conversely, some applications don’t lend themselves well to threadless mechanical linear actuators unless you take special precautions:

• Heavy direct or side load situations on the driveshaft itself are not good. You can get systems with special linear-bearing options to take the load off the shaft.

• In general, look with caution at unclean environments. Dirt picked up on an exposed driveshaft can transfer to the roller system, causing damage like you might expect in any contaminated rolling-element bearing system. Special boots or bellows may help.

• Where positive drive is needed, that is, where you need high precision and repeatability, consider alternatives such as ball screws or precision lead screws. You might be able to use a threadless linear actuator by adding servo control with positive feedback and motor control, but that complexity could negate the system’s basic simplicity.

Application hints

Life. Threadless mechanical linear actuators typically are rated much like ball screws and linear rolling-element bearings: for life in inches of linear travel. In general, manufacturers size units and recommend selection procedures that offer life spans in millions of inches. Unless you have specific life in mind, choose a system that can provide 1 million to 100 million in. of travel.

• Lead. Bearings support the driveshaft and the skewed rollers. The fewer the number of turns those bearings make for a given travel, the longer the system life. Therefore, higher leads promote longer life.

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• Overload. An advantage of most threadless linear actuator systems is that they can slip during occasional thrust overload and drive again as soon as the overload subsides. In fact, one manufacturer calls its offering a “linear clutch” for that reason. However, frequent or extended periods of slippage could lower life. Use the slip as a means of safety, but don’t design-in continual slippage as characteristic regular operation, for example — clamping.

• Environment. We already mentioned the need for cleanliness of the actuator’s surroundings. The need stems mostly from the many places of a threadless linear actuator that have rolling surface contact. The shaft itself may be supported on bearings. Bearings also support the rollers of the nut. The roller-to-shaft surfaces are natural places for wear, too. The bearings may be factorylubricated. Some manufacturers recommend lubricating the shaft; others do not. Regardless, it is important to keep the shaft clean. Be careful that a lubricant doesn’t defeat itself by carrying contaminants into unprotected places.

• Temperature. If you expect uncommonly high temperatures, say above about 160 F, check with the manufacturer. Standard bearing greases may not tolerate much more than that, but special high-temperature greases may be available.

Backdriving. Most threadless mechanical linear actuators can backdrive: The normally driven component can become the driver. While some situations welcome this as an attribute (for example, an operator may wish to move an unpowered unit manually during setup), in other situations it can be a danger if you aren’t prepared for it. Though most threadless actuators can run in vertical shaft orientation be aware that, like a ball screw, the load might run back during a power failure. Don’t rely on a threadless actuator to hold position. If it is important to process or personnel safety, use a true fail-safe brake.

Not just any shaft. Manufacturers supply shafting to go with their threadless linear actuators, and that should be your safest bet. However, if you choose to make your own, or you need an instant replacement or refurbishment, be aware that you can’t just throw an old broom handle in there and make it work. Shaft hardness is important because of the rolling contact. Manufacturers generally call for hardness of about 56 to 60 Rc. Check first. Also, the surface should be ground, providing a uniform friction surface. You may need stainless steel to help keep the surface uniform if corrosion could be a problem, but be sure that its hardness is adequate.

Stroke length, of course, is not really unlimited — for especially long systems you could run into practical problems of how to butt lengths of shaft together seamlessly. For very large slenderness ratios (shaft length to shaft diameter) you must also consider critical speed — the speed at which the shaft could vibrate or “strum” at its natural frequency. But, in general, stroke lengths of 10 to 16 ft or less are readily accommodated, and manufacturers can work with you for longer systems.

Typically, the threadless mechanical linear actuator has a reported mechanical efficiency of about 90%, which puts it well ahead of Acme type lead screws and in the same ballpark with ball screws. Also, most manufacturers report backlash in the range of 0.001 in. or less.